1. Field of the Invention
The present invention is directed to processing a metal substrate intended for use as a medical device enclosure.
2. Prior Art
U.S. Pat. No. 5,673,473 to Johnson et al. relates to a “method of forming an implantable electronic device having shield portions that are formed after the finishing by a bead-blasting step and joining the shield portions to form a hermetic enclosure containing the electronic circuit.” (See the examiner's reasons for allowance dated Feb. 26, 1997; underline added for emphasis.) In other words, the claims of the '473 patent are directed and limited to drawing and forming a metal shield AFTER beading blasting the metal. Moreover, Johnson et al. assert at column 1, lines 56 to 59 that their invention “performs the process steps on a continuous roll of titanium sheet stock, prior to forming, eliminating the need for subsequent processing steps.”
In addition to disclosing its explicit method for surface finishing an enclosure for an implantable medical device, Johnson et al. refer to the prior art of U.S. Pat. No. 5,113,681 to Guesnon et al. This patent discloses a conventional “process and apparatus for forming a sheet element with contoured surface from a plane titanium alloy sheet by hot deep drawing process.” Guesnon et al. describe their process at column 1, line 50 to column 2, line 28 as one “for fabricating contoured-surface sheet elements, particularly cylindrical or conical, that is simpler and faster and which nonetheless produces elements of regular thickness and precise dimensions with no cracks. The forming process . . . is characterized by the plane sheet element being preheated to a temperature of at least 730° C., by the preheated element being transferred to a non-preheated deep-drawing tool placed in the press soon enough for the temperature of the element to be still at least 700° C. at the time of the deep-drawing operation, by transferring the deep-drawn element to a calibration tool and heating the deep drawn element to at least 650° C. for a sufficient time for its definitive shaping, then allowing the so-processed element to cool to room temperature. The process of the present invention is particularly suitable for forming a sheet element 25 mm thick or less, and the deep-drawn element is preferably kept at a temperature of at least 650° C. in the calibration tool for at least one hour.”
“For forming such a sheet element . . . the temperature of the deep-drawn sheet element is brought to about 650° C. in at least 5 hours and held at about 650° C. for approximately one hour, allowed to cool in the calibration tool for at least 10 hours, and then is removed from this tool and allowed to air-cool.”
“The formed sheet element may . . . be descaled by shot-blasting and, for this purpose, steel balls with diameters between 0.6 millimeters and 0.16 millimeters are used for shot-blasting.”
“After shot-blasting . . . the element is finished by being pickled in a cold bath of an aqueous solution with 15 to 40 wt. % nitric acid and 1 to 2 wt. % hydrofluoric acid, with the nitric acid/hydrofluoric acid weight ratio being higher than 10 and the solution containing less than 10 g/l of the total (iron+titanium) for 1 to 5 minutes followed by rinsing and drying.”
“If, after forming, blasting, and the finishing stage of chemical pickling, the element is left in storage for a relatively long period of time, a patina forms which negatively affects its appearance but not its properties. It can then be chemically pickled in a cold bath similarly to the finishing stage of chemical pickling.”
Johnson et al. found Guesnon et al.'s process and other conventional processes deficient for conditioning the metal in the roll form or after the rolling process at the mill. In particular, Johnson et al. state at column 1, lines 33 to 53 that:
“Surface finishing and bead-blasting of implantable medical device enclosures in particular, however, has traditionally been performed during the device enclosure manufacturing process subsequent to formation of the device enclosure itself. This is because suppliers of titanium have been unwilling to invest in additional specialized processing equipment necessary for bead-blasting material in the small quantities generally required by implantable medical device manufacturers. Another reason that surface finishing of implantable medical device enclosures has traditionally been performed at the finished device level is to eliminate any scratches or blemishes that have marred the device enclosure at earlier points in the enclosure manufacturing process. Therefore, while the surface finishing processes employed in the manufacture of titanium implantable medical device enclosures have been generally successful in providing devices having scratch-free surfaces, it is desired to develop a material finishing process that maintains a blemish-free surface while eliminating the necessity for the post-forming finishing processes normally associated with manufacturing of implantable medical device enclosures.”
While the formed metal materials of the Johnson et al. and Guesnon et al. patents are completely acceptable for use as medical device enclosures, there is still a need for a drawn metal article having a uniform surface finish and with a sufficient surface roughness for bond ability during subsequent assembly operations. The present invention provides this.